Servo-controlled variable-displacement reversible hydraulic motors are frequently used to actuate secondary aircraft control surfaces, such as wing leading edge and trailing edge flaps, leading edge slats, door systems and the like, because of their hydraulic power efficiency. Such motors are typically connected to the aircraft's pressure supply line and to the return line. In such an arrangement, servocontrol of motor displacement determines the magnitude and direction of the motor output torque. These motors are typically supplied with a constant hydraulic supply pressure, and are controlled to produce a desired output torque by means of adjustment of a tiltable swashplate to change the motor displacement. In a reversible motor, the swashplate can be tilted in either direction through a zero-displacement centered position to produce both positive and negative torque. This capability is often referred to as “over-center” control.
In a typical application, the driven control surface, when deflected outwardly into an airstream, imposes a spring-like load. In general, the driving torque increases with outward surface deflection into the airstream. However, when the control surface is returned from such deflected position, the energy in the aerodynamic spring must be dissipated in the hydraulic system. In presently known systems, fluid from the motor pressure port is driven back into the pressure supply line and the motor return port draws fluid from the return line. Both of these conditions are undesirable as contamination may be introduced into the pressure supply system, and the return line may cavitate. A preferable arrangement under this condition would allow fluid from the pressure port to be bypassed directly to the motor return port.